Looks like it's memory issue..May be high pressure on CPU

1)Check max memory set .If not better to maximum memory

2)Threads unable to create for connections...check sp_configure

max worker threads

It should be dynamic value..It should not the fixed Value

Please refer below link you may get solution :

http://blogs.msdn.com/b/sqlsakthi/archive/2011/07/06/error-18056-state-29-the-client-was-unable-to-reuse-a-session-the-failure-id-is-29-after-upgrading-sql-2000-to-sql-2008-sp2.aspx

Thanks

Asr